As recording media for recording information such as writings, sounds or images, recording media have heretofore been known which employ e.g. a phase change material made of a magnetic material, a magnetooptical material, an organic dye material or an inorganic material, for a recording layer.
Such recording media are designed so that a physical parameter of the recording layer, such as the refractive index, electrical resistance, shape, volume or density, is changed by imparting an external factor to the recording layer, such as heating of the recording layer by irradiation with light or conducting an electrical current, or application of a magnetic field to the recording layer. And, such recording media are usually designed to carry out recording or readout of information by utilizing the difference in such a physical parameter value as between before and after imparting the external factor to the recording layer.
An example of such recording media is optical recording media whereby recording is carried out by irradiation of a laser beam. Such optical recording media may further be generally classified into a write-once-read-many optical recording medium which is only once writable and not rewritable, and a rewritable optical recording medium which is repeatedly rewritable. Among such optical recording media, a write-once-read-many optical recording medium has attracted attention in recent years for such a reason that it is suitable for recording e.g. official documents in which no change of information is permitted, it is suitable for high speed recording, or the production cost can be made low.
For the write-once-read-many optical recording medium, various types such as a phase-change type, an alloy type and a perforated type employing inorganic material, or ones employing organic compounds, have been proposed. Among them, a type as disclosed in JP-A-4-298389, wherein an inorganic substance capable of discharging a gas is incorporated in a thin recording layer, and a gas is discharged under heating by laser irradiation so that recording is carried out by a deformation thereby formed, is prospective, since it is possible to take a large signal amplitude, and secure high irreversibility to prevent rewriting.
According to JP-A-4-298389, if an inorganic substance which is decomposed by heating to discharge a gas, such as silver oxide or iron nitride, is used as a recording layer, a gas will be discharged under heating by laser irradiation, and voids will be formed by the discharged gas or recesses will be formed at the interface of the recording layer by the pressure of the discharged gas. And, by the formation of such voids or recesses, the optical constants or optical conditions such as the light path length at the irradiated portion will be changed, and the reflectance will be lowered, whereby a large signal amplitude can be obtained.